Assessment Of Urban Forest Ecological Benefits In The Semi-arid Region

The urban forest ecosystem plays an irreplaceable role in maintaining the ecological balance of the city.The evaluation of the ecological benefits of the urban forest is helpful for citizens to intuitively realize the importance of the urban forest and the supporting role of ecological land for the urban ecological environment.Lanzhou is a typical river valley city,which is located in semi-arid area and its ecological environment is fragile.As the artificial urban forest ecosystem of Lanzhou,the north and south mountains are the important ecological barriers.By evaluating the ecological benefits of the north and south mountains,it can provide reference for urban planning and ecological land protection and utilization in Lanzhou.Selecting the southern and northern mountains of Lanzhou City as research region,using land use data of the three periods(1994,2006,2017)of the Google Earth Engine cloud platform,random forest method was adopted to extract the land use/cover data in 1994,2006and 2017.Based on the extraction of land surface parameter data,landscape pattern index method was used to analyze the change of landscape pattern in the study area.Carbon sequestration capacity and Habitat Quality of the study area were evaluated by In VEST model Carbon module and Habitat Quality module respectively.The resource scarcity coefficient,the willingness to pay coefficient and the spatial heterogeneity coefficient were introduced to modify the equivalent factor method,and the ecological benefits of the study area were quantified.The main conclusions of the study are as follows:(1)From 1994 to 2017,the main land types in the north and south mountains of Lanzhou were grassland and farmland,followed by shrub(garden),construction land and bare land,and woodland and water area were the least.The area of construction land,woodland,grassland and shrub(garden land)showed an increasing trend,while the area of farmland and bare land showed a decreasing trend.Reservoir and the construction of urban park water body,making the water area slightly increased.At the level of patch type,the patch fragmentation degree of farmland and bare land increased and the dominance degree decreased.The patch fragmentation degree of construction land,woodland,grassland,shrub(garden)and water area showed a decreasing trend,and the dominance degree increased.At the level of landscape type,spatial heterogeneity decreased,diversity and dominance increased,spatial distribution tended to be uniform and regular,the degree of fragmentation of patches gradually decreased,the degree of agglomeration and connectivity increased,and the ecosystem function gradually enhanced.(2)In 1994,2006 and 2017,the carbon storage in the northern and southern mountains of Lanzhou was 74.03×10~6MGC,76.63×10~6MGC and 77.32×10~6MGC,respectively.From1994 to 2017,the amount of carbon sequestration increased by 4.44%,and the carbon sequestration capacity showed an increasing trend.The area of carbon sink and carbon source region is gradually decreasing,while the area of carbon balance region is increasing.The spatial distribution of carbon storage has obvious spatial differentiation characteristics of high in the southern mountains and low in the northern mountains.(3)The mean habitat quality of the northern and southern mountains in Lanzhou in1994,2006 and 2017 was 0.38,0.43 and 0.45,respectively.The habitat quality of poor and poor grades showed a decreasing trend,while the habitat quality of medium,good and excellent grades showed an increasing trend.The spatial distribution characteristics of habitat quality values changed from high in the southern and low in the northern mountains to high in the northern mountains and low in the southern mountains in the later period.(4)In 1994,2006 and 2017,the ecological benefit value of the north and south mountains was 280 million yuan,333 million yuan and 336 million yuan,respectively.From1994 to 2017,the total increase was 557.05×10~5yuan.In terms of ecosystem category,the contribution rate of grassland was the highest,reaching about 65%,while the contribution rate of construction land was negative.In terms of the category of ecosystem services,the contribution rate of regulatory services was the highest,reaching about 71%.From the perspective of profit and loss matrix,the conversion of farmland,bare land and construction land to grassland,and the conversion of grassland to woodland and shrub are the main reasons for the increase of ecological benefit value,while the conversion of water area and construction land slow down the growth rate of ecological benefit value.The ecosystem service value has obvious spatial differentiation characteristics,which are higher in the south mountains and lower in the north mountains.There is a certain aggregation characteristics,and the spatial heterogeneity is obvious.